1. Field of the Invention
The present invention relates to a support for the automotive exhaust gas purification catalyst or the combustion catalyst.
2. Description of the Prior Art
The prevailing catalyst for the treatment of combustion gas and automotive exhaust gas includes transition metals, transition metal oxides, and noble metals each supported on a proper support. The catalyst support is required to be able to support the expensive catalyst components in the highly dispersed state for their effective use. For this reason, the catalyst support is required to have a large surface area and to be stable at high temperatures for a long time.
One of the inexpensive catalyst supports to meet these requirements is transformative alumina having a large surface area. The well-known disadvantage of transformative alumina is that it transforms to .alpha.-alumina upon heating at high temperatures above 1000.degree. C., resulting in a substantial decrease in surface area. In the case where the transformative alumina is used as a catalyst support in the form of pellets or coating on molded products, the transformation to .alpha.-alumina and the structure change resulting therefrom cause the coating layer to fall and promote the sintering of the catalyst component. It is known that transformative alumina can be heat-stabilized for the prevention of surface area decrease by the addition of an alkaline earth element or rare earth element. (Japanese Patent Laid-open Nos. 117387/1979 and 14600/1973 and U.S. Pat. No. 4,021,185)
The incorporation of an alkaline earth element or rare earth element alone is not enough to impart sufficient heat resistance to alumina which is used as a support of the combustion catalyst (for a gas turbine or automobile) which is exposed to high temperatures above 1000.degree. C. The most heat resistant support developed so far is one which is incorporated with lanthanum as a rare earth element. It retains its surface area of 20 m.sup.2 /g after heating at 1200.degree. C. for 4 hours, but this is not enough. The present invention was completed to solve the above-mentioned problems involved in the prior art technology.